Air-cooled engines are forcefully cooled by cooling air sent to a cylinder head and a cylinder block from a cooling fan that is driven by a crankshaft. This type of air-cooled engine is disclosed in Japanese Laid-Open Patent Application No. 2-275021 and Japanese Examined Utility Model Application No. 58-19293.
In the air-cooled engine disclosed in Japanese Laid-Open Patent Application No. 2-275021, an intake valve and an exhaust valve are opened and closed as a result of a camshaft being rotated by a crankshaft via a power transmission mechanism. In this air-cooled engine, the combustion chamber in the cylinder head and the cylinder in the cylinder block are cooled by cooling air sent from the cooling fan to the cylinder head and the cylinder block. In order to improve the efficiency of cooling with this cooling air, it is preferable that the cooling air be conducted to the vicinity of the combustion chamber and the cylinder.
However, the power transmission mechanism is disposed on the side of the cylinder head and on the side of the cylinder block. Therefore, a compartment for accommodating the power transmission mechanism is disposed in the vicinity of the combustion chamber and the cylinder. This compartment is an obstacle to the cooling air being conducted to the vicinity of the combustion chamber and the cylinder.
In order to resolve these problems, in the air-cooled engine in Japanese Laid-Open Patent Application No. 2-275021, the effects of cooling the cylinder are improved by providing part of the compartment with an air duct for allowing the passage of cooling air.
The need has also increased for techniques whereby cooling air can be more actively conducted to the vicinity of the combustion chamber and the cylinder to further improve the effects of cooling the combustion chamber and the cylinder.
The air-cooled engine disclosed in Japanese Examined Utility Model Application No. 58-19293 is an inclined-cylinder engine having a base on the bottom of the crank case, and also having a cylinder block and cylinder inclined to the side of the crank case. The air-cooled engine can be mounted on any other arbitrary member by using bolts inserted through mounting holes in the base.
Also, the outer periphery of the cylinder block has a plurality of cooling fins extending in a direction perpendicular to the axial line of the cylinder. In this air-cooled engine, the cylinder can be cooled by the flow of cooling air among the plurality of cooling fins.
The casing for the air-cooled engine is often a cast article wherein the crank case, the base, and the cylinder block are integrated in order to reduce manufacturing costs. When the casing is manufactured by casting, the metal mold is opened along the cooling fins after the molten metal in the cavity of the metal mold has solidified. However, since the cylinder block and cooling fins are inclined in relation to the base, the direction in which the metal mold opens is different from the orientation of the mounting holes of the base. When the casing is being cast, the mounting holes cannot be formed simultaneously. After the casing is cast, the mounting holes must be mechanically worked in. This places a limit on improving the productivity of the casing.
One method for solving these problems is to provide the metal mold with a separate sliding die, and to form mounting holes by using this sliding die. This method allows the mounting holes to be formed at the same time as the casing is being cast. However, the structure of the metal mold becomes complicated with this method because a sliding die is provided to the metal mold.
In view of this, the need has arisen for techniques whereby the mounting holes can be formed at the same time that the casing is cast and whereby the configuration of the metal mold can be simplified.